Explanation: A video amplifier in a radar system processes signals that are intended for direct display on the screen. * **Low level video (D)** is the primary input, representing the amplified radar echoes from targets. This signal is amplified by the video amplifier to drive the display. * **Fixed range rings (A)** and **Variable range rings (B)** are electronically generated markers that indicate distances from the radar. These signals are typically mixed with the target video *before or within* the video amplifier stage so they appear on the display along with the target echoes. A **resolver signal (C)**, however, provides angular position information, usually indicating the precise bearing of the antenna. This signal is used by the radar system to synchronize the display sweep with the antenna's rotation, ensuring targets are shown at their correct bearing. It *controls* the display's sweep, but it is not itself a video signal to be amplified and displayed.